The U.S.—the most volcanically active country after Russia—has 169 volcanoes that the USGS and other agencies regularly monitor. These volcanoes are rated based on eruption frequency and the risks posed to nearby populations, infrastructure, and air traffic, among other factors.

Eighteen of the U.S. volcanoes on the list are deemed "very high threat," with only Hawaii's Kilauea volcano surpassing Mount St. Helens, according to the latest USGS report, released in 2005.

Although the May 18, 1980, eruption and resulting landslide destroyed much of the volcano's summit area, Mount St. Helens still stands 8,364 feet (2,549 meters) above sea level.

If Mount St. Helens reawakened violently, an ash plume reaching 30,000 feet (about 9,100 meters) or more could materialize in as little as five minutes, grounding aircraft and wreaking havoc on agriculture, water and power supplies, and human health, Ewert said.

Glaciers atop the mountain could also melt and carry volcanic debris down the valley.

For nearby communities—including Seattle and Portland, Oregon—the layers of volcanic ash settling out of the sky would be "just pretty darn unpleasant," he added. "Imagine someone dumping an inch of talcum powder on your house."

Mount St. Helens a "Sneaky" Volcano

Still, Mount St. Helens's 1980 eruption—which killed 57 and flattened more than 200 square miles (518 square kilometers) of forest—"really launched modern volcanology in a lot of ways. That's one of its lasting legacies," Ewert added.

"When you consider you can have a volcanic reawakening and move into dangerous eruptions in a few weeks, if you don't have your act together and have instruments in place ... you're literally flying blind," Ewert said.

"With so many people and infrastructure at stake, we felt like that was a roulette game we're uncomfortable with as scientists."

That's why several U.S. volcano observatories support a National Volcano Early Warning System, which is now working its way through Congress. If put in place, the national network would ensure that the most dangerous U.S. volcanoes are being monitored, for example with seismographs and sensors installed on their flanks.

Right now only about a hundred of the 1,545 known active volcanoes worldwide are outfitted with monitoring equipment, noted Rick Wunderman of the Smithsonian Institution's Global Volcanism Program in Washington, D.C.

Despite an ongoing "revolution" in monitoring volcanoes, the progress in predicting eruptions is still "surprisingly slow," said Wunderman, who worked at the Cascades Volcano Observatory near Mount St. Helens not long after its 1980 eruption.

Mount Rainier Volcano Also One to Watch

While Mount St. Helens gets its share of attention, Wunderman added that another Washington State volcano is also one to watch—Mount Rainier, the third most dangerous volcano on the 2005 USGS list.

"The point is that [Mount Rainier] doesn't even have to erupt," he said. "It's so big, so tall, and has so much ice that an earthquake or landslide" could combine with the region's heavy rainfall to send floodwaters gushing into valley communities built on an ancient floodplain below.

"It's a much more bleak picture than St. Helens in the sense of runoff—that's one thing that's on people's minds."

"Every now and then galvanizing eruptions occur. It isn't often that they're so big or anything of that sort, it's more the media and the consciousness of people who are taxpayers, or stuck people in an airport," he said.

"So it was [with] St. Helens, which went eight hours straight at full throttle ... and rained down over a vast area of the West—it woke people up."